Paint on concrete at certain spots contained a compound that conducted electricity, causing false signals and forcing removal. But with 99% of construction done, the line could open in November.

It was to be a little aesthetic touch added to the Eastside extension of the Gold Line as it neared completion.

But no one imagined what gremlins would be unleashed when workers added a layer of paint to the concrete at "cross-over" points where the light-rail trains could switch tracks.

The coloring agent was made of iron oxide. And at intersections like 1st and Clarence streets in Boyle Heights it caused the painted concrete to conduct an electrical circuit that basically told a lie.

"It was sending out a false signal that the train was there," said Dennis Mori, the Gold Line Eastside extension's project manager. "When the weather got hot, it did it more. . . . When we removed the painted concrete, the false signal disappeared."

As the Metropolitan Transportation Authority works to replace the painted concrete, the glitch is just one reminder of the challenges faced in completing the county light-rail system's first leg into the Eastside. For a while, officials estimated that the six-mile line could open sometime in the summer, maybe as early as June.

Now they are hoping for a November opening. That will still give the MTA enough time to do testing and open the extension before the year is over, and a federal funding deadline arrives.

On the surface, it looks as if there's quite a lot of work to do. On a stretch of the line along 3rd Street in East L.A., bulldozers and cranes excavate asphalt, and stretches of track lie exposed after the colored concrete was removed. Workers shovel in asphalt, which will be painted, but without conducting false signals.

There are shallow trenches along the side of the track. But Mori said most of the work that remains is finishing touches, and testing.

"We're 99% done with construction," Mori said.

The roughly $890-million Gold Line extension, which runs from Union Station to Atlantic Boulevard, has 1.7 miles of twin tunnels and two underground stations. But the bulk of it runs down the center of comparatively narrow streets, leaving not much separation between the line and vehicles passing by.

In contrast, much of the Pasadena-to-downtown L.A. portion of the Gold Line was built along train right of ways, Mori said.

For the Eastside extension, a vigorous safety campaign has begun, including citing people who jaywalk across tracks.

"People have to adapt to a rail line in the center of the street," said Frank Villalobos, whose firm Barrio Planners is the lead architect for the project. "I've been involved since the beginning, and I think a lot of things that would be considered impediments have been overcome."

The challenges have included fault lines miles underneath Soto Street and the unearthing of the skeletal remains of more than 108 people just outside Evergreen Cemetery. Most of the remains are believed to belong to Chinese laborers buried in a potter's field in the late 19th and early 20th centuries. A school also had to be moved.

"I'd rather they take their time and make sure everything is safe for the community," said Diana Tarango, 74, a lifelong Boyle Heights resident and member of the MTA's resident advisory committee for the Eastside extension.

There has been much anticipation on the Eastside about the line. Residents have long complained that the area has a large number of mass transit riders but no rail service.

Although the extension itself is relatively short, it gives Eastside residents a route into downtown and to Union Station, where more rail routes are available.

There has also been talk of a Phase 2 pushing the line farther east.

Tarango said she can't wait for the Gold Line trains to rumble through the neighborhood. She said she still remembers the disappointment she felt when it was decided that the Red Line subway would not come to the Eastside in the 1990s.

"We've been waiting a long time for this," Tarango said. "I think it's going to be a whole changing of the community. I really believe it."